Abstract The adiabatic quantum-flux-parametron (AQFP), which can operate with ultra-low energy consumption, has attracted attention as an alternative to semiconductor circuits. The development of reconfigurable AQFP devices is essential for advancing AQFP integrated circuits. However, research on reconfigurable devices at the logic gate level remains limited. We propose an AQFP circuit capable of reconfiguring its logic function by controlling the superconducting phase shift through externally applied magnetic flux. In AQFP logic, logic ‘1’ and ‘0’ are represented by the polarity of the current. The proposed circuit can switch between the Buffer function, which propagates the input current with its polarity unchanged, and the NOT function, which propagates it with the polarity inverted. We analytically derived the parameter conditions for correct operation and determined the parameters within this range to achieve adiabatic operation and sufficient element margins for experimental demonstration. We fabricated the proposed reconfigurable circuit using the 10 kA cm −2 Nb four-layer superconducting circuit fabrication process, and experimentally confirmed correct operation for both the Buffer and NOT functions. The control current margins, which are responsible for the functional reconfiguration, were −2.3% to +31.2% for chip 1 and +0.8% to +35.1% for chip 2. Both margins had widths comparable to the simulation result. This work presents the first experimental demonstration of an AQFP circuit whose function can be reconfigured using only a control current. We believe this work will contribute to the advancement of reconfigurable AQFP devices.
Abstract The adiabatic quantum-flux-parametron (AQFP), which can operate with ultra-low energy consumption, has attracted attention as an alternative to semiconductor circuits. The development of reconfigurable AQFP devices is essential for advancing AQFP integrated circuits. However, research on reconfigurable devices at the logic gate level remains limited. We propose an AQFP circuit capable of reconfiguring its logic function by controlling the superconducting phase shift through externally applied magnetic flux. In AQFP logic, logic ‘1’ and ‘0’ are represented by the polarity of the current. The proposed circuit can switch between the Buffer function, which propagates the input current with its polarity unchanged, and the NOT function, which propagates it with the polarity inverted. We analytically derived the parameter conditions for correct operation and determined the parameters within this range to achieve adiabatic operation and sufficient element margins for experimental demonstration. We fabricated the proposed reconfigurable circuit using the 10 kA cm −2 Nb four-layer superconducting circuit fabrication process, and experimentally confirmed correct operation for both the Buffer and NOT functions. The control current margins, which are responsible for the functional reconfiguration, were −2.3% to +31.2% for chip 1 and +0.8% to +35.1% for chip 2. Both margins had widths comparable to the simulation result. This work presents the first experimental demonstration of an AQFP circuit whose function can be reconfigured using only a control current. We believe this work will contribute to the advancement of reconfigurable AQFP devices.
Adiabatic quantum-flux-parametron (AQFP) logic is a promising technology for future energy-efficient, high- performance information processing systems because it has significantly low power consumption due to the adiabatic switching of Josephson junctions. We are developing a high-performance field-programmable gate array (FPGA) using superconducting AQFP circuits to reduce its energy consumption. The all-AQFP FPGA consists of logic blocks, switch blocks, connection blocks, and memory using AQFP circuits. In particular, the memory is composed of AQFP buffer chains, enabling high-density and low-power memory. The switch blocks can perform data routing as well as several logic functions, including majority,and, andorfunctions, which increases the design flexibility of the all-AQFP FPGA. We fabricated a one-unit all-AQFP FPGA and demonstrated its reconfigurable operation at low speed. It was found that much lower power consumption can be achieved in the new FPGA than in the other superconducting FPGAs.
Abstract Adiabatic quantum-flux-parametron (AQFP) is an adiabatic logic based on the quantum-flux-parametron. It has extremely low switching energy, as well as high signal gain and high robustness. In this work, we present the development of an AQFP cell library using the MIT LL 100 μ A μ m −2 superconductor electronics fabrication process, SFQ5ee. Four types of basic sub-cells, i.e. buffer, inverter, constant, and branch, are designed as the building blocks for combinational AQFP logic cells. Taking advantage of the 8 metal layers, we place the dc superconducting quantum interference device (SQUID) part of a buffer above the ground plane and the signal transformer part below the ground plane to mitigate unwanted couplings. In addition, by vertically overlapping the signal transformer and the dc SQUID, the dimension of a buffer is reduced to 15 μ m × 20 μ m, which is only 30% of that of the conventional design. More complex AQFP logic cells, such as the majority gate, AND gate, and OR gate, are designed by arraying the sub-cells together. Correct operations and wide excitation current margins are confirmed for the established AQFP logic cells by both simulation and experiments.
Abstract Adiabatic quantum-flux-parametron (AQFP) logic is an energy-efficient superconductor logic with zero static power and very small dynamic power due to adiabatic switching operations. In order to build large-scale digital circuits, we built AQFP logic cells using superconductor magnetic shields, which are necessary in order to avoid unwanted magnetic couplings between the cells and excitation currents. In preliminary experimental tests, we confirmed that the unwanted coupling became negligibly small thanks to the superconductor shields. As a demonstration, we designed a four-to-one multiplexor and a 16-junction full adder using the shielded logic cells. In both circuits, we confirmed correct logic operations with wide operation margins of excitation currents. These results indicate that large-scale AQFP digital circuits can be realized using the shielded logic cells.
Adiabatic quantum-flux-parametron (AQFP) logic devices can operate under extremely low bit energy approaching the thermal limit by varying their potential energy adiabatically using ac bias currents. We have proposed an on-chip ac power source, which is composed of a Josephson relaxation oscillator and a superconductingLCresonator in order to drive AQFP circuits at 5 GHz or higher frequencies. We herein experimentally confirmed the correct switching operation of an AQFP gate at 4.94 GHz using the proposed on-chip ac power source. Furthermore, we confirmed that the ac power source can drive approximately 800 AQFP gates.
Abstract We have been developing adiabatic quantum-flux-parametron (AQFP) circuits as an ultra-low-power superconductor logic for energy-efficient computing. In a previous study, we proposed and demonstrated a quantum-flux-latch (QFL), which is a compact and compatible latch for AQFP logic. The QFL is composed of an AQFP buffer gate and a storage loop, which are directly connected to each other. However, the operation margins were not sufficiently wide due to a trade-off between the operation margins of the storage loop and that of the buffer gate. In this present study, we propose a magnetically coupled QFL (MC-QFL), where the storage loop and the buffer gate are physically separated and magnetically coupled to each other to eliminate the trade-off in the operation margins. The simulation results showed that the critical parameter margin of the MC-QFL is twice as large as that of the previously designed QFL. For comparison, we fabricated and demonstrated both the previously designed QFL and the newly designed MC-QFL. The measurement results showed that the MC-QFL has wider operation margins compared with the previously designed QFL.
Abstract Adiabatic quantum-flux-parametron (AQFP) logic stands as an energy-efficient superconductor logic family relying on ac excitations. Though a four-phase clocking scheme has been commonly applied to excite and clock AQFP circuits, the cost of multiple sources and long between-stage latency (a quarter clock cycle) remain unsatisfactory. These drawbacks are more severe for sequential logics, where feedback loops require both down-flow and reverse-flow AQFP gates, necessitating extra buffers and imposing a limit on operating depth at the same time. To overcome these restrictions, in this paper, we introduce a novel mixed clocking scheme for low-latency AQFP logic circuits allowing feedback loops. In particular, the down-flow part of the AQFP circuit is excited by an ac current through delay-lines, which can reduce the latency from input to output stages. Meanwhile, the reverse-flow part is clocked by 2 other ac currents with 90° phase difference to reduce the buffering gates. The proposed clocking scheme is first applied to a fundamental logic circuit, confirming correct functionality and low-latency operations for diverse feedback conditions. For experimental validation, we design and fabricate a 3-bit AQFP counter using the proposed clocking scheme based on an on-chip clock splitter. The circuit is examined at 4 GHz under 4.2 K temperature, where we obtain desired operations and wide excitation margins.